Prior art dog leash clip patents go as far back as year 1895, when Isaac Joyner of Anderson Ind. was granted a U.S. Pat. No. 548,062 that pictured a snap hook very similar to the dog leach clips used today. The clip was not precisely described, but the drawing is very similar to today's production clip. U.S. Pat. No. 5,462,019 by Hong-Rong, in 1995 had a clear description of the prior art clip, line 53 through line 61, column 2.
There are two features of the prior art clip that cause a problem. The position of your fingers for opening the clip creates excessive friction on the plunger, making the overall required force excessive. With continued use, the friction increases, resulting in even higher resistance to opening the clip.
A second problem occurs in cold climates. Water can collect inside the clip and freeze. Ice inside the clip prevents opening the clip and releasing the dog collar, until the ice has melted.